A typical differential odometer includes a complementary pair of sensors. Each of these are mounted near an opposing wheel of a vehicle. Typically these wheels are non-drive powered. For instance, in a front wheel drive vehicle, the sensors are mounted on the opposing left and right rear wheels. Each sensor generates a series of output pulses when the wheel, associated with the respective sensor, rotates. This series of output pulses is further processed by computation to determine vehicular distance traversed and heading, or differential distance traversed. Finally, with an absolute vehicle reference, a dead reckoned position for the vehicle can be determined.
The accuracy of information regarding vehicle heading is important in navigation systems because it effects the accuracy of the dead reckoned position. Because inaccuracies of the sensors are inherent, provision for calibrating these sensors is important. If calibrated accurately, after combining the results of the two sensors and determining distance traversed, each wheel should indicate identical distance traversed if the vehicle is traversing a straight line.
Calibration of the differential odometer in modem navigation systems is a manual process. Also it is done with the intervention of a vehicle operator. The conventional method for calibrating wheel sensors requires that an operator drive the vehicle over a straight path, carefully indicating to his navigation system when he comes to the start and finish points on the path. The system can count the number of pulses received from each wheel sensor between the start and finish points, and calculate the distance traversed per pulse for each wheel. Then, the system can scale the output of each wheel sensor such that the resulting distance traversed and heading indications are corrected, or calibrated for inherent errors.
This conventional calibration method is prone to operator errors, and it is not always possible for the operator to find a straight path having a known length with easily discernible start and finish points. Further, if the path is incorrectly measured, or if the operator did not properly align the vehicle on the path before indicating the start of the path, incorrect calibration may result.
Also, with aging of the system and its components, or with changing environmental operating conditions, the erroneous behavior of the wheel sensors is dynamic. For instance this may include a difference of tire size because of a slow leak or misbalanced inflation. Similarly, if an operator changes to his smaller spare the differential odometer will be inaccurate. This may be important to an operator who wants to find a service facility while relying on his navigation system. Since these errors can be substantial, without dynamic, or continuous, calibration, the vehicle heading, thereby the dead reckoned position can be substantially erroneous.
What is needed is a more accurate, and robust differential odometer calibration method that is continuous and automatic in nature.